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These ferns may be the first plants known to share work like ants

At the top of the forest, a mass of strange ferns grabs a tree trunk, resembling a giant tangle of chipped antlers and floppy disks. Beneath these leaf fronds and closer to the core of the lush knot are disc-shaped brown plants. They are also ferns of the same species.

Ferns – and possibly similar plants – can form a complex, interdependent type of society that was once considered limited to animals such as ants and termites, according to online researchers May 14 in Ecology.

Kevin Burns, a biologist at Victoria University of Wellington in New Zealand, first became acquainted with ferns while doing fieldwork on Lord Howe Island, an isolated island between Australia and New Zealand. He took note of the local epiphytes (plants that grow on other plants) and one of the species caught his eye: the Staghorn fern (Platycerium bifurcatum), also native to parts of mainland Australia and Indonesia.

“I realized, God, you know, they never occur alone,” Burns says, noting that some of the largest groups of ferns were massive groups of hundreds of individuals.

It soon became clear to Burns that "each of those individuals was doing a different thing."

He compares fern colonies with an upside-down umbrella made of plants. Ferns with long green and waxy “strap” fronds seemed to divert water to the center of the aggregation, where the brown, spongy disk-shaped “nest” fronds could absorb the water.

A colony of common ferns (Platycerium bifurcatum) grows in the canopy of the forest on a cedar tree on the island (Guioa coriacea), its brown, absorbent nesting fronds at the bottom and the nucleus of the colony and green-striped fronds projecting towards the exterior.Ian Hutton

The shrubbery reminded Burns of a pile of termites, with a community storehouse of resources and the segregation of different jobs in the colony. Scientists call this type of cooperative groups, where overlapping generations coexist and form castes to divide labor and reproductive, “eusocial” roles. The term has been used to describe certain societies of insects and crustaceans, along with two species of mole rats as the only examples of mammals (SN: 18/10/04). Burns wondered if ferns could also be eusocial.

Analysis of his team on the fertility of fronds revealed that 40 percent could not reproduce and members of the barren colony were predominantly nest fronds. This suggests a reproductive division of labor between nest and strap frond types. Frond absorption tests confirmed that nest fronds suck more water than leash fronds. Previous research by other scientists has found root networks that stretched across the colony, meaning nesting fronds have the ability to relax the thirst of the fronds. The fronds divided the work, as did the ants and termites.

The team also analyzed genetic samples from 10 colonies on Lord Howe Island and found that eight were composed of genetically identical individuals, while two contained ferns of different genetic origins. High degrees of genetic relationship are also observed in eusocial insect colonies, where many sisters contribute to nest survival.

Taken together, Burns thinks these traits mark many of the boxes for eusociality. That would be a “big deal,” he says.

An assumed requirement for eusocial colonial life is behavioral coordination, because it allows different individuals to work together. But ferns are plants, not animals, that so often coordinate their behaviors. Seeing eusocial living in plants “seems to me to indicate that this kind of transition in the evolution of complexity does not require a brain,” Burns says.

The study opens up the “opportunity to look (epiphytes) through the lens of eusociality,” which is “very cool,” says Michelle Spicer, an ecologist at Puget Sound University in Washington who did not participate in this study.

Spicer points out that the exchange of water and nutrients is known in other epiphytic plants. However, Burns notes that the division of labor to build community resources "appears to be a key feature that distinguishes staghorn (ferns) from other colonial plants."

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A stressful life in the canopy – away from the ground – may have contributed to the evolution of fern eusociality by providing security in water and nutrients, Burns says.

“The epiphytic lifestyle certainly makes group life easier and that’s where all social stories begin,” says Brian Whyte, an evolutionary biologist at the University of California, Berkeley, who is not involved in this research.

These ferns could certainly fit into the definition of eusociality, says Whyte. He is particularly fascinated by how plants form castes and colonies in the wild but remain as individual fronds when grown in the ground as ornamental plants. This variability differs from many eusocial species, according to him.

Burns and colleagues are currently investigating whether strap fronds can become nest fronds after being transplanted to another part of the colony. Burns also wants to study another species of staghorn ferns in Madagascar that seems to grow in colonies as well.

Whyte sees important benefits in broadening the vision of eusociality to include plants.

"It's great to be able to notice something and be like, 'Wait, this is comparable to some of the coolest, most advanced societies in the living world,'" he says. intriguing with the animals that form caste. "Learning more about (these ferns) will improve our theories about why these characteristics evolved in the diversity of life."

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